1. Field of the Invention
The present invention relates generally to non-destructive testing of objects for defects, and, more particularly, to an eddy-current test instrument providing compensation for variations in spacing from a workpiece surface.
2. Related Art
Eddy-current test instruments having a rotary head have been used for some time in the non-destructive testing for defects of materials. Nearly equally long, at least since the introduction of non-sliding test probes, has been the application of spacing compensation means for such devices (U.S. Pat. No. 3,281,667; German patent application 1,773,501). Such distance compensation has been found necessary, as otherwise a reasonable evaluation of the defect signals conforming to the depth of the defect would not have been possible, since defect signals are subject to substantial variations in sensitivity because of the unavoidable distance variations.
Early spacing compensation means employed controllable amplifiers having a non-linear control characteristic as the primary control elements. It was necessary, however, that the characteristic curve have a specific shape, in order to accommodate the fact that the distance signal and the sensitivity variation of the defect signals relates differently to the probe distance from the workpiece. Compromises, therefore, had to be accepted with respect to obtainable control accuracy. Also, replacing probes having different characteristics was problematic, if not impossible.
These problems have been partially overcome by distance compensation relying on a digital control system having a linearly controllable signal amplifier and the combination of the distance functions was achieved by adequately dimensioned resistance chains (German patent publication 2,530,816).
An important disadvantage, however, of the techniques used up to now is that for each test head offset mounted on a circular track, a complete distance compensation with distance probe, distance signal processing and network is required for the combination of the distance functions. This is based on the fact that the sources for distance variations (e.g., eccentricity and out of roundness of the part to be tested) have different effects at any given time for each point of the circular track, and, therefore, for each of the offset mounted test heads. As with rotary heads, there is always required a signal transmission from rotating to stationary parts, and there is also required for the distance signals of each test head its own transmission channel (e.g., in the form of a rotary transmitter). Because of the large number of transmission channels required anyway, this latter requirement is very often hard to achieve.